I want to tee my stdout to a log file from inside the script. And maybe stderr too.

Let's start with the simplest case: I want to tee my stdout to a logfile, as well as to the screen.

This means we want two copies of everything that gets sent to stdout -- one copy for the screen (or wherever stdout was pointing when the script started), and one for the logfile. The tee program is used for this:

# Bash
exec > >(tee mylog)

The process substitution syntax creates a named pipe (or something analogous) and runs the tee program in the background, reading from that pipe. tee makes two copies of everything it reads -- one for the mylog file (which it opens), and one for stdout, which was inherited from the script. Finally, exec redirects the shell's stdout to the pipe.

Because there is a background job that has to read and process all our output before we see it, this introduces some asynchronous delay. Consider a case like this:

# Bash
exec > >(tee mylog)
echo "A" >&2
cat file
echo "B" >&2

The lines A and B that are written to stderr don't go through the tee process - they are sent directly to stderr. However, the file that we get from cat is sent through our pipe and tee before we see it. If we run this script in a terminal, without any redirections, we're likely (not guaranteed!) to see something like this:

~$ ./foo
A
B
~$ hi mom

There is really no way to avoid this. We could slow down stderr in a similar fashion, hoping to get lucky, but there's no guarantee that all the lines will be delayed equally.

Also, notice that the contents of the file were printed after the next shell prompt. Some people find this disturbing. Again, there's no clean way to avoid that, since the tee is done in a background process, but not one that's under our control. Even adding a wait command to the script has no effect. Some people add a sleep 1 to the end of the script, to give the background tee a chance to finish. This works (generally), but some people find it more offensive than the original problem.

If we avoid the Bash syntax, and set up our own named pipe and background process, then we do get control:

There's still a desynchronization between stdout and stderr, but at least it no longer writes to the terminal after the script has exited:

~$ ./foo
A
B
hi mom
~$

This leads to the next variant of this question -- I want to log both stdout and stderr together, keeping the lines in sync.

This one is relatively easy, as long as we don't care about destroying the separation between stdout and stderr on the terminal. We just duplicate one of the file descriptors:

# Bash
exec > >(tee mylog) 2>&1
echo A >&2
cat file
echo B >&2

In fact, that's even easier than the original question. Everything is synchronized correctly, both on the terminal and in the log file:

~$ ./foo
A
hi mom
B
~$ cat mylog
A
hi mom
B
~$

There is still a chance of part of the output coming after the next shell prompt, though:

~$ ./foo
A
hi mom
~$ B

(This can be solved with the same named pipe and background process solution we showed before.)

The third variant of this question is also relatively simple: I want to log stdout to one file, and stderr to another file. This is simple because we don't have the additional restriction that we must maintain synchronization of the two streams on the terminal. We just set up the log writers:

And now our streams are logged separately. Since the logs are separate, there's no concern about the order in which the lines are written between them. However, on the terminal, we will get mixed results:

~$ ./foo
A
hi mom
B
~$ ./foo
hi mom
A
B
~$

But some people won't accept either the loss of separation between stdout and stderr, or the desynchronization of lines. They are purists, and so they ask for the most difficult form of all -- I want to log stdout and stderr together into a single file, BUT I also want them to maintain their original, separate destinations.

In order to do this, we first have to make a few notes:

If there are going to be two separate stdout and stderr streams, then some process has to write each of them.

There is no way to write a process in shell script that reads from two separate FDs whenever one of them has input available, because the shell has no poll(2) or select(2) interface.

Therefore, we'll need two separate writer processes.

The only way to keep output from two separate writers from destroying each other is to make sure they both open their output in append mode. A FD that is opened in append mode has the guaranteed property that every time data is written to it, it will jump to the end first.